1. Field of the Invention
This invention relates to the detection of SCSI (small computer system interface) devices installed at illegal locations on a SCSI bus. In particular, the invention relates to detecting in a mixed environment of eight bit SCSI devices and sixteen bit SCSI devices the eight bit SCSI devices installed at SCSI bus address-identified locations intended for sixteen bit SCSI devices.
2. Description of Related Art
First generation SCSI devices were eight bit devices and were addressed on a SCSI bus with an address ID of 0 through 7. With the advent of sixteen bit SCSI devices and larger peripheral systems particularly storage systems, the address ID for SCSI devices expanded from 0 through 7 to 0 through 15. With both sizes of SCSI devices available, narrow (8 bit) and wide (16 bit), computer users often mix narrow and wide SCSI devices as they upgrade their system.
In a peripheral system with mixed narrow and wide SCSI devices, a problem of conflicts in address may occur if the SCSI devices are not properly installed in the system. A narrow SCSI device will only correctly recognize it is being addressed if it is located at an address slot with an ID between 0 and 7. If a narrow SCSI device is installed in a slot with an address ID of 8 to 15, the narrow device will not recognize the high order ID address bit. Accordingly, a narrow SCSI device installed at ID's 8 through 15 will configure itself to respond to a SCSI address line for ID 0 through 7. For example, a narrow device installed at slot ID 8 will define itself as a SCSI device ID 0. When a controller activates the ID 0 address line, it will get a response from a SCSI device actually located at slot 0 and a conflicting response from the narrow device at slot 8 that thinks it is an ID 0 SCSI device. This will create bus conflicts and corrupt data.
This problem can be avoided if the user only installs narrow SCSI devices in slots 0 through 7. However, it frequently happens that the user incorrectly installs a narrow device in slots 8 through 15. In other words the narrow SCSI device is at an illegal location and has an illegal ID. The problem is how to detect SCSI devices installed at illegal locations.